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Ch 5. Molecules and Compounds Chemical Formulas of Compounds 1 Chem 30A Ch 5. Molecules and Compounds Chemical Formulas of Compounds 1. A given compound always has the same numbers and types of atoms (constant composi;on). 2. Each atom is represented by its element symbol. 3. The number of each type of atom is indicated by a subscript to the right of the element symbol (Excep;on: If just one atom, no subscript). 4. Typically, the more metallic element is listed first. Ex Probs Molecular Compounds and Ionic Compounds Types of Compounds Molecular Compounds Ionic Compounds • Covalent bond • Ionic bond • One unit: Molecule • One unit: Formula unit O H H + – Chemical Bond Chemical bond: A force that holds groups of atoms together and makes them func;on as a unit. • Bonding involves just the electrons of atoms! Molecular (Covalent) Compounds • Held together through Covalent Bonds: Bonds in which electrons are shared between the bonded atoms. H H H2 • Molecule: a group of two or more atoms which are covalently bonded (a unit of molecular compound) • Nonmetal + Nonmetal à Molecular Compound Molecules H H O H H Cl Cl C C O H H O Compounds Elements • Note: Both compounds (eg, H2O) and elements (eg. O2, H2) may exist as molecules. Representing Molecules CH4 Chemical formula H Ball-and-s;ck model H C H H Structural formula Space-filling model Ionic Compounds • Held together through Ionic Bonds: Bonds in which oppositely-charged ions are held together by electrostac arac;on between them. + – • Formula unit: A group of two or more ions which are bound through ionic bonds (a unit of ionic compound) • Metal + Nonmetal à Ionic Compound Formula Units • Formula units exist as a part of a larger 3D array (a solid). Ionic Compounds • Most commonly, ionic compounds form when metals combine with nonmetals à Metal element transfers electrons to the nonmetal element. Metal caon + Nonmetal anion à ionic compound Isolated ions don’t exist! Formation of an Ionic Compound Overview: Fundamental Components of Matter Matter Pure Substances • Elements • Compounds “Par;cles” of Maer • Atoms • Ions • Molecules Elements, Compounds, Atoms, Molecules, Ions Elements Compounds (1 type of atom) (more than 1 type of atom) • + – Atomic • Ionic • Molecular • Molecular Elements, Compounds, Atoms, Molecules, Ions Ex Probs Formula (Molecular) Mass Formula (Molecular) Masses of Compounds Formula mass of a compound = sum of atomic masses eg. Molecular mass of H2O = 2(atomic mass of H) + 1(atomic mass of O) = 2(1.008 amu) + 1(16.00 amu) = 18.02 amu Ex Probs Chemical Nomenclature Chemical Nomenclature • Chemical Nomenclature: a system of naming chemical substances Common Name Systemac Name Formula Grain alcohol Ethyl alcohol CH3OH Rubbing alcohol Isopropyl alcohol C2H5OH Laughing gas Dinitrogen oxide N2O Milk of Magnesium Mg(OH)2 magnesia hydroxide Epsom salt Magnesium MgSO47H2O sulfate heptahydrate Chemical Nomenclature A. Ionic Compounds (Binary and Polyatomic) B. Covalent Compounds (Binary) C. Acids Formulas (NaCl) Names (sodium chloride) Refer to Chemical Nomenclature Handout for rules for naming and wri;ng formulas. Naming Compounds Naming Binary Ionic Compounds • The caon is always named first and the anion second. • Remember: Ionic compound has a net charge of 0. • METAL + NONMETAL = ionic compound Naming Binary Ionic Compounds: Fixed Charge Caon Ionic compounds containing fixed-charge metals (mostly main group metals): 1. A monatomic caon has the same name as that of the parent element. 2. A monatomic anion is named by taking the root of the element name and adding –ide. • Examples KCl potassium chloride MgBr2 magnesium bromide CaO calcium oxide Ex Probs Naming Binary Ionic Compounds: Variable Charge Cations Ionic compounds containing variable-charge caons (mostly transi;on metals): For metals that can form caons of more than one charge, the charge on the metal ion must be specified with Roman numeral in parentheses! • Examples CuBr Copper(I) bromide FeS Iron(II) sulfide PbO2 Lead(IV) oxide Common Ion Charges (Memorize!) Metals form caons. Nonmetals form anions. • Groups 1A,2A,3A: • Group 5A: charge = -3 • Group 6A: charge = -2 charge = group # • Group 7A (halogens): • Transi;ons metals: charge = -1 Many have variable • Group 8A (noble gases): charges. charge =0 Ex Probs Naming Ionic Compounds with Polyatomic Ions Ions can be simple (monatomic) or polyatomic. • Simple Ions: Mg2+, Li+, Cl-, O2- • Polyatomic Ions: + - NH4 NO3 N N + - NH4 NO3 Naming Ionic Compounds with Polyatomic Ions Names and charges of polyatomic ions must be memorized. (Memorize the following!) • Polyatomic caons: Names end in -ium. + + NH4 = ammonium H3O = hydronium • Polyatomic anions: Many contain oxygen. – 2– OH = hydroxide CO3 = carbonate – 3– NO3 = nitrate PO4 = phosphate 2– – SO4 = sulfate CN = cyanide Common Polyatomic Ions Naming Ionic Compounds with Polyatomic Ions Examples of ionic compounds with polyatomic ions • NH4Cl ammonium chloride • Na2CO3 sodium carbonate • Mn(OH)2 manganese (II) hydroxide • (NH4)2SO4 ammonium sulfate Ex Probs Naming Binary Molecular (Covalent) Compounds NONMETAL + NONMETAL Name first element first, second element second. 1. First element: Greek prefix + parent element (Excep;on: Don’t use “mono-”). 2. Second element: Greek prefix + parent element + “-ide” (as if it were an anion). Naming Binary Covalent Compounds Examples of Binary Molecular Compounds N2O5 dinitrogen pentoxide P4O6 tetraphosphorus hexoxide BF3 boron trifluoride NO nitrogen monoxide CO carbon monoxide Ex probs Naming Problem Which of the following compounds is/are named incorrectly? a) KNO3 potassium nitrate b) TiO 2 ;tanium(II) oxide c) Sn(OH)4 ;n(IV) hydroxide d) PBr 5 phosphorus pentabromide e) CaCrO 4 calcium chromate Strategy for Naming Compounds Look at the formula. Metal + Nonmetal Nonmetal + Nonmetal or Polyatomic Ions MOLECULAR IONIC Compound Compound Name as Caon + Anion! Use Greek prefixes! (If see variable-charge ca*on, include charge in Roman numerals!) Ex probs Writing Formulas Writing Chemical Formulas: Ionic Compounds I. Ionic Compounds § Balance charges, since compounds must be neutral! Posi;ve Charge + Negave Charge = 0 Examples • magnesium bromide MgBr2 • iron(II) sulfide FeS Ex probs Writing Chemical Formulas: Molecular Compounds II. Binary Molecular Compounds § Greek prefixes indicate number of atoms. Greek Prefixes Examples mono one di two • dinitrogen pentoxide N2O5 tri three tetra four • boron trifluoride BF3 penta five hexa six Ex probs Strategy for Writing Formulas of Compounds Look at the name. Metal + Nonmetal Nonmetal + Nonmetal or See Greek prefixes Polyatomic Ions COVALENT compound IONIC compound Follow Greek Balance charges! prefixes! Ex probs Nomenclature for Acids Brief Intro to Acids and Bases Arrhenius Defini;on of Acids and Bases • Acid: a substance that makes proton (H+) in water. HCl à H+ + Cl- acid proton anion • Base: a substance that makes hydroxides (OH–) in water. + – NaOH à Na + OH Brief Intro to Acids and Bases Bronsted-Lowry Defini;on of Acids and Bases • Acids are proton (H+) donors. HCl à H+ + Cl- acid proton anion • Bases are proton (H+) acceptors. + + NH3 + H à NH4 base proton caon Strong and Weak Acids • Strong acid: Completely ionized (dissociated). HCl(aq) H+(aq) + Cl-(aq) • Weak acid: Most of the acid molecules remain intact. + - CH3COOH(aq) H (aq) + CH3COO (aq) equilibrium arrow Naming Acids *Acids always have “H” first in their formulas! End all acid names with “acid.” 1. Binary Acids: Name format: “hydro-” + parent element + “-ic acid” Naming Acids 2. Acid with oxygen (Oxyacids): Acid name is derived from oxoanion of that acid. • If anion ends in “ate” à acid name ends with “ic” acid. – § NO3 is nitrate à HNO3 is nitric acid • If anion of ends in “ite” à acid name ends with “ous” acid. – § NO2 is nitrite à HNO2 is nitrous acid Some Oxyacids Common Acids (Memorize!) Strong Acids • HCl hydrochloric acid • HBr hydrobromic acid • HI hydroiodic acid • HNO3 nitric acid • H2SO4 sulfuric acid • HClO4 perchloric acid Weak Acid • HC2H3O2 or CH3COOH ace;c acid • H2CO3 carbonic acid Ex probs .
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